[aerohead]

For what it's worth,
I ran across an online paper by Douglass et al., from the University of Delaware, USA, in which they were still using the 10=5 rule-of-thumb as of 2006.
Hucho has something close to it in 1986. It's a bit more liberal for Diesel-powered vehicles.
William H. Bettes, used it in his researches in the Graduate Aerodynamic Laboratory California Institute of Technology ( GALCIT ) wind tunnel, reporting in 1982.
General Motors' Aerodynamics Laboratory was using it in the early 1990s.
'Highway' is not to be construed with 'EPA HWY' protocols.